This invention relates to a test signal attenuation circuit, and more particularly, to a circuit to reduce transmitted power which will verify that proper Bit Error Rate (BER) performance can be achieved with reduced power.
Such a test tells the operator that the link will perform properly even after being subjected to environmental factors that reduce the transmitter power, degrade receiver sensitivity, or attenuate the channel. An example of such a need is the pre-flight checkout of an aircraft data network, although the invention is useful with any network interface.
In many systems, a reduction in transmitter power is not possible or practical. For example, many commercially available off the shelf (COTS) electro-optic transmitters are logic-output based, with levels predetermined. Many gigabit data networks employ Emitter Coupled Logic or other logic with fixed voltage levels out. Such logic levels cannot easily be reduced for health monitoring purposes or to determine whether the system or circuit will operate at reduced voltage or power levels. Even where such power control is available, it is often a single, discrete level change, rather than a continuous adjustment. The circuit or system designer has no flexibility regarding the amount of attenuation that is to be introduced into the data link. In addition, the approach of opening connectors to the link so as to provide access to the link through a special test equipment may create contamination problems, particularly in case of fiber optic links and does not check the transmitter or receiver itself, or any section of the cable between the transmitter and receiver. Also, special test equipment is often not available in field operations.
This invention therefore relates to a built-in test signal test attenuation circuit to determine whether the system will operate satisfactorily. Since it is built into the transmitter circuitry, it is always available. Further, since it relies on pulse-width modulation of the data pulses to degrade the link, rather than signal amplitude attenuation, it works with any link.